BP has ordered clean-up contractors in the Gulf of Mexico to pay out wages to workers suspected of claiming for work they have not done, according to sources close to the companies involved.

In an interview with the Guardian, Rear Admiral James Watson, the federal on-scene co-ordinator for the oil spill, admitted that there had been "instances of fraud".

BP is responsible for paying the wages of an estimated 27,000 people who have signed up to work in the Gulf on the clean-up operation after the explosion on the Deepwater Horizon rig in April. Classed as "volunteers" by BP, they work for myriad government agencies and contractors. BP's bill for the whole operation stands at more than $2bn (£1.3bn), and the vast majority of this covers wages.

Workers are paid daily, depending on what needs to be done. The majority work every day but BP officials said that on some days fewer workers were needed.

A BP factsheet states: "There are limitations for the type of work volunteers can perform", with certain tasks requiring special training.

The lowest-skilled workers are those who clear beaches of seaweed, driftwood and other debris to make it easier to clean the sand once oil reaches the shore. They earn $10 an hour. Those working for engineering contractors – building sand berms, for example – earn much more.

An attorney advising some of the contractors said that BP had told them to pay workers for the days they claimed to have been on site. The contractors were told not to quibble over the claims even if they suspected they were bogus.

Watson downplayed the size of any bogus claims. He told the Guardian at a New Orleans town hall gathering on Wednesday night: "I did hear some instances of fraud but it's not been too serious. Everyone is working for some sort of supervisor – you'd not last for too long [as a bogus claimant]." BP said: "We don't typically discuss publicly business guidance we give to our contractors. We're working with them across the Gulf states to mount an effective, comprehensive response to the Deepwater leak."

BP set up a body this week to handle its response to the spill, to be run by Bob Dudley, called the Gulf Coast Restoration Organisation.

Last night the judge who this week overturned the six month moratorium on deepwater exploratory drilling in the Gulf refused an appeal by the White House to delay the ruling. Rig operators – production operations are not affected – have received letters from the interior department saying the ban is no longer in place. But it is not clear when drilling will resume while other legal appeals are pending.